The invention relates to the general field of integrated circuit manufacture with particular reference to thin film inductors.
In the manufacture of integrated circuits incorporation of inductors (as opposed to capacitors) has generally been avoided because of the difficulty of fabricating them. Inductors are generally thought of as three-dimensional objects hence their unsuitability for integrated circuits. However, the basic formula for calculating the inductance value L of a particular coiled geometry is
L=(xcexcN2A)/s
where N is the number of turns in the coil, A is the mean cross-sectional area of the coil, s is the length of the coil, and xcexc is the magnetic permeability of the medium in which the coil is immersed.
In the macro world, inductors are usually formed by winding wire around a cylinder of fixed radius, thereby guaranteeing fixed cross-sectional area. More than one layer of wire turns are generally used, thereby increasing the value of N while keeping the value of s low. Instead of a cylindrical geometry a-spiral such as shown in FIG. 1a may be used. Spiral 11 is wound in a plane and has an inner starting point 12 and an outer ending point 13 both of which being used to contact the spiral (see example of lower level wiring 14 which appears in FIG. 1b which is an isometric view of FIG. 1a). However, the effective cross-sectional area (for determining an inductance value) of such a spiral will be less than the actual cross-sectional area of the full spiral. This is offset to some extent by the fact that the length (s) of the spiral coil is significantly reduced relative to that of a cylindrical coil, even allowing for edge effects.
Thus, spiral inductors have proven popular for use in integrated circuits even though the magnetic permeability xcexc of the medium in which the coil is immersed is unity. In a macro coil of cylindrical design, xcexc can be increased to a much higher value than that of air by inserting a core of a material such as soft iron in the interior of the cylinder, said core having a diameter only slightly less than that of the coil itself.
Another factor in thin film inductor design that needs to be mentioned is that, because of the close proximity of all the components to one another, stray lines of magnetic flux associated with the inductor can have an effect (mutual inductance) on nearby components and devices. This is often hard to predict and unexpected side effects associated with inductors in integrated circuits are an ongoing problem.
A routine search of the prior art was conducted but, as far as we have been able to determine, no attempts have been made in the prior art to increase the permeability associated with a thin film inductor or to reduce unexpected proximity effects. For example, Abidi et al. (U.S. Pat. No. 5,539,241) describe a thin film inductor which is formed in a manner such that it is suspended over a pit in the substrate. This reduces parasitic capacitance thereby raising the self resonant frequency of the inductor
Lue (U.S. Pat. No. 5,863,806) describes how an inductive coil that is three dimensional and therefore occupies less area, maybe formed.
Desaigoudar et al. (U.S. Pat. No. 5,370,766) show how a thin film inductor may be formed as a byproduct of other process steps so that the additional cost of having an inductor in the circuit is reduced to a minimum. Desaigoudar et al. (U.S. Pat. No. 5,450,263) is a divisional of the previous patent, claiming the structure.
It has been an object of the present invention to provide a thin film inductor having high inductance per unit area.
Another object of the invention has been to increase the magnetic permeability of the medium in which a thin film inductor is immersed.
Still another object of the invention has been to provide a low reluctance path for the magnetic flux associated with said inductor, thereby reducing inductive effects on neighboring components and devices during circuit operation.
These objects have been achieved by adding to a spiral inductor a core of high permeability material located at the center of the spiral. If the high permeability material is a conductor care must be taken to avoid any contact between the core and the spiral. If a dielectric ferromagnetic material is used, this constraint is removed from the design. Several other embodiments are shown in which, in addition to the high permeability core, low reluctance paths have been added to the structure. In one case this takes the form of a frame of ferromagnetic material surrounding the spiral while in a second case it has the form of a hollow square located directly above the spiral.